Caring for Filter Presses

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Question:

Steve, I run a job shop that performs metal cleaning, phosphating and painting of metal. Since we did not generate very much wastewater for a number of years, we would collect and thicken our wastewater pretreatment sludge and then have it disposed off-site. However, due to our growth in business recently, our wastewater volume significantly increased, and we decided to buy and install a small, used filter press to dewater the sludge in order to reduce handling and disposal costs. However, from day one, when we opened the press thinking that it is full, we find only the bottom third of the chamber is full and the rest of the filter cloth covered with only a thin layer of sludge. Our feed pressure on the feed diaphragm pump is 90 psig, and we air blowdown the press for at least 30 min. We have cleaned the press with a pressure washer, and it did not help. Any suggestions? J.T.

Answer:

Since I cannot actually see your filter press in operation or the filter press cake (sludge is such a “dirty” word) that you generate, I cannot pinpoint a solution, but let me offer several ideas for you to try.

First, you state that your filter press feed pump’s pressure is 90 psig. While this is a good final pressure, I do not recommend that this is the pressure you start. I recommend that you incrementally increase the air pressure to your diaphragm pump in four stages: 25-35 psig; 45-55 psig; 70-80 psig and 90-95 psig. By starting out at a low pressure, you minimize the likelihood of “smashing” the floc particles against the filter cloth and causing premature clogging or blinding of the filter cloth. My rule of thumb is, if the time period between pulses of the air diaphragm pump is greater than 30 sec, it's time to increase pressure to next stage, and if you are already at 90-95 psig, it is time to stop the feed pump and turn on your air blowdown.

Second, you did not mention if you turn off the filter press feed pump before the filter press is “full.” Sometimes when you stop the filter press feed pump in mid-cycle, the filter cake will drop to the bottom of the chamber as the feed pressure subsides.

Third, you may want to examine your pump. Since you purchased the filter press used, the pump may be too small for the press, or it needs to be rebuilt. I would contact the filter press manufacturer, get its recommendation for the size of the air diaphragm pump, and see if your pump is the same or a larger size as recommended.

We had a similar problem at one of our client’s; and it was solved after the pump’s air handling mechanisms were totally rebuilt with new parts. Apparently the pump, due to wear, was not efficiently transferring the air line pressure to the pumped sludge. To check this out, you can install a pressure gauge into the center feed pipe of the filter press and compare it to your air line pressure when the pump is stalling.

Fourth, since you purchased a used filter press, the filter press cloths may be the wrong type or its weave may be too tight. Most filter press owners do not realize that filter press cloths not only come in a number of types and finishes, but also come in various degrees of tightness. For the vast majority of metal hydroxide sludges, a polypropylene felt cloth with no special finish is usually okay. Regarding tightness, filter cloths for the dewatering of metal hydroxide sludges typically have a rating of 5-10 cfm Frazier. Again, contact the filter press manufacturer or filter cloth suppliers for their recommendation (see Products Finishing’s 2003 Directory and Technology Guide, filter presses or visit the Suppliers page at www.pfonline.com/suppliers). For a small fee, some suppliers offer bench scale testing to determine the best filter cloth.

Lastly, you may just have a waste stream that easily binds up the filter cloth. You mentioned that you perform metal cleaning and painting. If you have a burn-off oven for the removal of paint, this could be the source of your problem. Paints contain a very fine powder, titanium dioxide. Since the paint resins are burned off in this oven to remove the paint, what remains is a fine ash. If you find that this is your problem, you will, at a minimum, need to precoat your filter press with diatomaceous earth (DE) and, possibly, body feed a DE slurry into the filter press while you are feeding your sludge. The filter press manufacturer can help you design a simple DE slurry feed system and give you its recommended DE dosage for precoat or body feed. Try the precoat first, since in the vast majority of cases this solves the problem. Most chemical suppliers should have access to DE, but if all else fails, you can purchase DE at your local pool supply store.

One needs to keep in mind that the filter press cloth does not really perform any filtering; it provides a base upon with the incoming feed material can build upon, and as the filter press cake forms, it filters itself. Hope the above ideas help you solve your problem.